2 BULLETIN OF THE 



Dr. Douglas Houghton, in his first Report on the Geology of Michi- 

 gan, remarks upon the " appearance of primary and trap rocks forming 

 mountahi chains, and the great disturbance which has taken place 

 since the deposition of the red sandstone," and says that this sandstone 

 in the vicinity of Granite Point is "scarcely disturbed, resting upon 

 nobs of primary rocks." * In Dr. Houghton's Fourth Annual Keport, 

 for 1841, t the rocks of this region are described as primary ones, con- 

 sisting chiefly of granite, sienite, sienitic granites, and greenstone with 

 metamorphic rocks on their flanks, forming a stratified series consist- 

 ing of " talcose, mica and clay slates, slaty hornblende rock, and quartz 

 rock ; the latter rock constituting by far the largest proportion of 

 the whole group." He considered that the granite passed "almost in- 

 sensibly into a serpentine rock." (/. c, p. 482.) In like manner, he 

 thought that the granites on the southeasterly side of the district 

 changed going northwesterly into a greenstone, and that the dikes trav- 

 ersing these granites were identical with the greenstone, having been 

 injected into the granite. His serpentine bears a close resemblance to 

 greenstone, and he states that " possibly a more close examination may 

 show it to be a simple series of dikes, lying parallel to the line of cleav- 

 age of the slate rocks." (/. c, p. 494.) Regarding Presque Isle he says : 

 " This point of land has its origin from the simple elevation of a mass of 

 trap rock, which rises on the north in abrupt cliffs, varying from twenty 

 to sixty feet in height. The trap is mostly greenstone, though portions 

 of it are so largely impregnated with a dark-colored, almost black ser- 

 pentine, as to deserve the name of serpentine rock. The knob of trap 

 luider consideration is possessed of additional interest, from the un- 

 equivocal evidence of uplift, as also from the manner in which these 

 evidences are exhibited. The cliff's of trap occupy the very extremity 

 of the point, while the neck and central portions are made up of con- 

 glomerate or trap tuff" and sand-rock, resting uiwn the trap. These 

 upper rocks also appear upon the immediate coast, in cliffs of from 

 twenty to sixty feet in height, and in many places they are seen resting 

 directly upon the trap. The stratification of these sedimentary rocks 

 has been very much disturl)ed, and they invarialdy dip, at a high angle, 

 in all directions from the trap itself The character of both rocks, at the 

 immediate line of junction, is almost completely lost, and the evidences 

 of change most unequivocally marked. But the most curious feature 

 of the wdiole is, that the sedimentary rocks, for a distance of several 



* History of Michigan, by J. H. Lannian, (New York, 18-39,) pp. 348, 352. 

 t Joint Documents, Michigan, 1841, pp. 471-607. 



