APPLICATION OF PRINCIPLE OF TRANSGRESSIVE OVERLAP 587 



the Manitou Springs region the basal sandstone is Lower Ordovicic rather 

 than Cambric. That continued encroachment of the sea caused the over- 

 lap of the Ordovicic is shown in a number of cases along the Front range. 



In the Sangre de Cristo range the Arkansas sandstone of Carbonic age 

 overlaps the Lower Paleozoics, resting for the most part directly on the 

 granite foundation of the range. This case, however, is probably not an 

 example of progressive overlap, but of the irregular type. 



Foreign examples. — The basal Paleozoic section of the north of Scot- 

 land furnishes a record of nearly continuous subsidence, and therefore of 

 progressive advance of the sea on the land of that period. Eesting un- 

 comformably on the pre-Cambric Torridon sandstone is a basal conglomer- 

 ate with pebbles up to an inch in diameter, made of the underlying ma- 

 terial. This passes upward into cross-bedded sandstones and arkoses, 

 which in turn grade upward into the "pipe rock," a fine quartzite pene- 

 trated by numerous worm tubes (Scolithes sandstone, Eriboll quartzite). 

 These basal elastics, probably in part non-marine, are from 450 to 600 

 feet thick, and are succeeded by mudstone, the so-called Fucoid beds, in 

 which calcareous sediment first appears. This is the beginning of the 

 granular dolomite which becomes most characteristic of the upper beds. 

 The dolomites, with a thickness of perhaps 1,500 feet (calciferous sand- 

 rock or Durness limestone series of Scottish geologists), ranges in age 

 from Cambric to Lower Ordovicic. The calcareous beds bear evidence 

 of accumulating in quiet water, yet there is near the middle of the series 

 a cross-bedded sandstone, which indicates an interruption and temporary 

 return of shore or dry land conditions, after which offshore sedimentation 

 again took place in this region. 



This section, then, indicates that a progressive subsidence took place 

 (interrupted by the interval referred to), and that hence we must look 

 somewhere for successive overlapping of the beds and the rise of the 

 basal elastics in the series. The record of overlapping has been destroyed 

 by erosion in the northern area, but in Wales we still find traces of it. In 

 southern Wales the basal beds of the Cambric are conglomerates, sand- 

 stones, and shales, with lower Cambric fossils, and 1,570 feet thick 

 (Caerfai group). They are succeeded by 1,800 feet of sandstones and 

 slates, with mid-Cambric fossils (Solva group), and higher still by 750 

 feet of shales and grits, with Paradoxide daudis and P. hicksii (Mene- 

 vian). These are followed by the Lingula flags (2,000 feet), and are 

 later succeeded by the Tremadoc slates (1,000 feet), which are regarded 

 by British geologists as forming the top of the Cambric, but are classed 

 by continental geologists with the basal Ordovicic. There is a record of 

 subsidence here, but the subsidence is not so marked, nor is the shore zone 



