76 The American Geologist. August. 1897 
site of a dike, "a small portion only remaining in the south end 
of the rent, to attest its former presence," The chasm, it has 
since been determined, Is undoubtedl^y due to the removal of 
more finely jointed rock lying between master joints in the con- 
glomerate. 
Jackson even extended his views of igneous causes to the 
explanation of gneiss, thus anticipating the conclusion though 
not otferrlng the basis for it. which later investigators have 
worked out with the aid of the microscope. He regards gneiss 
in one stage of his work as the rapidly cooled crust of granite. 
Mica-schist he admits may be of n)etamorphic origin, that is 
in the Lyelllan sense. In explaining the amphibolite of the 
pre-Cambrian area of Rhode Island, he states, "I am of the 
opinion that it derives its occasionall}' stratiform structure 
from an admixture of argillaceous slate rock through which 
it was elevated, or that the Jiornblende rock has partially 
fused and assimilated the superincumbent slates." But here 
he appears to have taken a hint from De la Beche, who held 
to a similar hj'^pothesis. 
In common with the elder Hitchcock, Jackson recognized 
in southern Rhode Island a group of clays and sands older 
than the very latest drift phenomena, but he included with 
these deposits the extensive sand-plains which occur about 
Providence. Under a mistaken notion as to tlieir origin, these 
beds were referred to the Tertiary period. It is now known 
that the sands and clajs pertain to a succession of glacial de- 
posits, the Columbia and succeeding deposits of the Pleisto- 
cene. 
In the matter of geological forces, he was not strictly a 
Strabonian. In speaking of the disintegration of rocks, he 
considers ''the causes formerly in action vastly more energet- 
ic than they are now.'' 
The Rhode Island report was not yet completed when Dr. 
Jackson was appointed state geologist of New Hampshire, 
Sept. 10th, 1839. By the law which authorized this appoint- 
ment, provision was made for one assistant, "who shall be a 
skillful analytical and experimental chemist." To this office 
J. D. Whitney was appointed in December, 1840, and served 
in the laboratory that winter. Subsequently other assistants 
were employed. Although Jackson and Whitney parted com- 
