GEOLOGY OF AROOSTOOK COUNTY, MAINE. 41 



brachiopods, together with a lamellibranchiate shell, resembling Cypficardinia. These 

 are not sufficient to determine the exact geological horizon, but taken in connection with 

 the other beds of the neighborhood, leave little doubt that they are Silurian. One mile 

 south of Ashland village is another exposure of limestones, but, in the absence of instru- 

 mental measurements, it is impossible to say in what way it is related to the beds already 

 described. It is, however, much more richly fossiliferous, and from it the following species, 

 determined by Mr. Ami, were obtained : — 



Favosites Gothlandicus, Lamarck. 

 Strophomena rhomboidalis, Wilckcns. 



" punctulifera, Conrad. 



Melista arcuata. Hall. 

 Platyceras, up. 

 Polypora Psyche ? Billings. 

 Murcbisonia, sp. 

 Loxonema Fitclii, Hall. 

 A Strom atoporoid form. 

 Crinoidal columns. 



These fossils indicate that the limestones in question, like those of Square Lake, are of 

 the horizon of the Lower Helderberg formation. They were so regarded by Prof Hitch- 

 cock, but by that author are further described as probably forming an anticlinal, upon the 

 slopes of which the higher beds are to be regarded as Devonian. Of this view our obser- 

 tions failed to aflbrd confirmation. Between Ashland and Presqu'ile admirable oppor- 

 tunities are afforded for the study of the rock-formations of the district, first along the 

 stage route connecting these two places, and secondly along the channel of Aroostook 

 River. The last-named section, being the most complete, will be here described, with only 

 incidental references to the former. 



At the mill on Big Machias River, a quarter of a mile above its junction with the 

 Aroostook, and half a mile from Ashland Bridge, are four exposures of grey, light-weather- 

 ing slates, which are so calcareous as to approach limestones in character, and from 

 which attempts have been made to obtain lime. Their dip is soixth-eastei-ly at an angle of 

 only twenty degrees. Similar beds are also exposed on the Aroostook itself, one mile below 

 the bridge, and near the mouth of the Little Machias, but here their dip in much higher, 

 being N. 60^ W. < *70-8.5^ "With these are beds of grey, buff-weathering, flaggy sandstones, 

 which appear to be a continuation of similar beds seen on the right bank of the Aroostook, 

 half a mile above the bridge, and which abound in plant remains, though too obscure to 

 admit of identification. The dip is the same at both points, and their position would 

 seem to be above that of the limestones just described. Between the two, however, it is 

 probable that a band of green and red slates, with thin beds of calcareous conglomerate, 

 intervenes, beds of this character appearing a mile or so further down the stream, in a 

 manner which seem to indicate that they hold this position. Their dip is comparatively 

 low, varying from N. 30° W. < 45° to N. < 30". Similar beds may also be seen crossing 

 the post-road between Ashland and Castle Hill. 



For three or four miles below the last described exposvires upon Aroostook River, 

 making a distance of about seven or eight miles from Ashland, the dip of the rocks becomes 

 reversed, being now south-easterly (S. GO-TO E. < 45° to 60"). They here consist largely of 



Sec. iv, 1887. 6. 



