Yol. 50.] STRUCTURE OF CARBONIFEROUS DOLERITES AND TUFFS. 605 



Observations in the field confirm the opinion expressed by the 

 Geological Surveyors. As additional evidence, there are two cases 

 in which we have a bed of tuff succeeded by a lava-flow, and of the 

 remaining 61 outcrops 11 prove to be tuffs. Of course in such 

 outcrops of the Toads tone as nos. 5 & 6, where the relative position 

 with regard to the limestone has not been exactly determined, it is 

 impossible to say whether the rock is intrusive or interbedded. I 

 have discovered no proofs of intrusion anywhere. 



The Toadstone is found in a district measuring about 25 miles 

 from north to south and 20 miles from east to west. The Geo- 

 logical Surveyors have mapped 60 outcrops, some of which are parts 

 of the same flow. 



The following Table (p. 606) gives a list of these outcrops. I have 

 applied to each a number, commencing from the north, and the names 

 of the chief places near which it passes, both for the purpose of this 

 paper and for future reference and identification. 



In column IV. the words ' Upper ' or ' Lower ' (with a cross 

 reference) mean that there is clear evidence that the two outcrops 

 in question are so related to one another, but it does not follow that 

 all the outcrops marked ' Upper ' are necessarily on the same 

 horizon. 



The specific gravity has been determined by a Walker's balance 

 made by How. Two specimens were also weighed with a chemical 

 balance. They gave 2 - 86, 2'87 with Walker's balance, and 2"876, 

 2-890 with the chemical balance. The determinations were there- 

 fore "016 and - 02 less by Walkers than by the chemical balance. 



The eighth column gives some idea of the comparative freshness of 

 the three principal minerals in the least altered specimen from each 

 outcrop of lava : thus / denotes that the mineral is fresh or un- 

 altered, and in the case of olivine that it is altered only along the 

 cracks ; a denotes that it has undergone a less or greater amount of 

 alteration, in some cases being entirely replaced by a pseudomorph. 

 In eleven out of forty-five outcrops fresh olivine is found, in twenty- 

 six fresh augite, and in thirty fresh felspar, and in ten all the 

 minerals are found in a fresh state. 



There are undoubtedly two beds at least which are recognized by 

 the Geological Survey, but whether they extend over the whole 

 district is uncertain. Two may be seen exposed on Chelmorton 

 Low, outcrops 26, 19 ; Matlock Bath, 39, 40 and 41, 41, 40 ; Ash- 

 ford, 31, 30 ; Lathkill Dale, 37, 36 ; Miller's Dale, 20, 19 ; Tides- 

 well Dale, 17, 9 ; Cressbrook Dale, 8 a, 8 6 ; on Weathery Low, 15, 

 14 ; and near Brook Bottom, 7 a, 7 b. 



Outcrops 13, 14, 25, 26, 27, 28, now separated by denudation, 

 were probably once connected and formed one bed, which would 

 be the lower one of the district about Miller's Dale. Nos. 17, 20, 

 24 might have belonged to this bed. Outcrops 9, 15, 19, and 

 perhaps 21 and 22 probably formed the upper bed of the district. 

 Two beds also appear at Ashford and in Lathkill Dale. There seem 

 to be, then, two well-marked beds or lava-flows near Miller's Dale 



