Wadsworth.] 
292 
[November 2, 
reasons that will appear later. Those who wish to compare the 
trachyte with the debris of the felsite, with which it has been con- 
founded, will be able to do so by examining the material at the 
base of the trachyte, or the same material on the shore lying 
between the massive felsites at the south-west side of Great 
Head (1218, 1219, 1220, 1221). The trachyte is a crystal- 
line homogenous rock, while the felsitic debris forms a heteroge- 
neous rock showing, even to the naked eye, its detrital character. 
Even the most highly altered portions seen in Eastern Massachu- 
setts never approach the trachyte in character, except superficially. 
My attention was first called to this trachyte by Prof. A. Hyatt, 
and later by Mr. W. O. Crosby. Mr. Crosby took this rock for a 
“ sandstone or arenaceous slate ” which he regarded as Carbonif- 
erous and later as Primordial. He held that the rock was chiefly 
composed of the felsitic debris, deriving that conclusion from the 
conglomerate at the base ; and remarked* that “ the removal of 
the sand-rock from the harbor, which it doubtless once filled, may 
have been the work of ice in recent geologic times ” 1 
Mr. Crosby’s conclusions, based on the supposition that the 
rock was a sandstone , are summed up in his “ Contributions to 
the Geology of Eastern Massachusetts” 2 as follows : “ I have else- 
where remarked upon the incontrovertable proof which this rock 
affords of the great antiquity of the depression in which it lies ; 
for obviously Marblehead Harbor was excavated from the border- 
ing Huronian diorite, petrosilex and granite, before the deposition 
of the sandstone now lying unconformably and horizontally upon 
its crystalline floor. So that although the removal of the sand- 
rock from the harbor, which it doubtless once filled, may repre- 
sent comparatively recent denudation ; yet the harbor itself must 
have had substantially its present form in early Palaeozoic times, 
having been since filled and re-excavated ” ( l . c., pp. 264, 265). 
This trachyte, the old rhyolites (felsites) and various other rocks 
seen in Eastern Massachusetts show, through their structure and 
their relations to other rocks, that eruptive phenomena have been 
very common here in past times. This action probably began 
1 American Naturalist, 1877, xi, 584, 585 ; Report on the Geological Map of Massachu- 
setts, 1876, p. 42; Am. Journ. Sci., 1880, (3) xix, 122. 
2 Occas. Pap. Bost. Soc. Nat. Hist., in. 
