Machen to Clark's father.

One of the earliest letters of the now 890 letters I’ve collected in research on Gordon H. Clark is one from J. Gresham Machen to David S. Clark in 1921. I’m not sure if the original is extent, but a photo copy of the letter is in the possession of Dr. Clark’s family. By the time of this letter Machen was already deeply involved in the Fundamentalist-Modernist controversy. He found in Clark’s father a Presbyterian minister on his side. It is likely that they knew each other personally, both living in or near Philadelphia and fighting for the fundamentalist cause in the PCUSA.
I’ve also found the review of David S. Clark’s which Machen refers to in the letter. I’ve transcribed it and posted on the Clark foundation website:
And the letter itself:
[Dr. J. Gresham Machen, Professor at Princeton University to Rev. David S. Clark, Pastor of Bethel Presbyterian Church, Phila., and father of Gordon H. Clark]

October 2, 1921

Rev. David S. Clark, D.D.,
My dear Dr. Clark:
I have read with the greatest interest your exceedingly able review of Professor Foster’s book in the “Presbyterian.” It is, of course, far more than a book review. Professor Foster represents the thinking of a large part of what is called, rightly or wrongly, “the religious world.” You have given an admirably clear exposition of the whole modernist position, and your criticisms go straight to the point. Particularly instructive is the way in which you are able to connect these present day popular tendencies with their roots in Schleiermacher and others. I feel greatly heartened by the presence of such a true defender of the faith in the pastorate in Philadelphia.
I hope I may see you before long.

Faithfully yours,

J. Gresham Machen