566 Correspondence — Mr. JR. Mallet. 



learn that it has lately been found at one of the Caldbeck Fell mines, 

 in Cumberland, associated with Pyromorphite, Anglesite, and various 

 other ores of Lead. As Molybdenite is rather common in some of 

 the adjoining mines, the occurrence of Molybdate of Lead might, 

 perhaps, have been expected, as a result of the decomposition of 

 Molybdenite and Galena. 



Another mineral new to the British list has just been detected in 

 the Haematite mines of the Cleator district. This is Hausmannite, 

 which occurs, well crystallized, in small pockets and veins associated 

 with Pyrolusite, mostly between the haematite and the limestone in 

 which it is found. 



Further notices of these minerals will appear in the Memoirs of 

 the Geological Survey. J. G. Goodchild. 



Penrith, 9th October, 1875. 



PRISMATIC STRUCTURE OF BASALT. 



Sir, —Assuming the description of the three basaltic prisms in the 

 collection of the Geological Society as given by Mr. Scrope to be 

 exact (see Geol Mag. 1875, Decade II. Vol. II. p. 412), the facts 

 do not in any way conflict with the explanation that I have given 

 of the mode of production of the lenticular cross-joints in basaltic 

 prisms. The prisms referred to must have come from that part 

 of the original mass in which occurred the dividing surface between 

 that part cooled from the top and that cooled from the bottom 

 of the mass, as is proved with respect to one of the prisms by the 

 existence in it of a joint having surfaces concave in both direc- 

 tions, such plane in fact passing horizontally through this articula- 

 tion ; other adjacent prisms may have their joints, within certain 

 limits above or below this plane, either convex upwards or down- 

 wards, for the slightest differences in the conductivity or conditions 

 and rates of cooling will suffice either to depress or elevate, by a 

 greater or less distance, the plane already spoken of. It is also not 

 difficult to see that several alternations in the directions of the con- 

 cave or convex surfaces may occur in the neighbourhood of the 

 meeting plane of cooling in opposite directions, where, as in the 

 case of other divergent or opposite heat waves, more or less confusion 

 in normal structure must occur. 



18th October, 1875. Eobt. MALLET. 



THE INVERTED STRATA OP THE MENDIPS. 

 Sir, — Eeferring to Mr. A. M'Murtrie's interesting paper " upon 

 certain isolated areas of Mountain Limestone at Luckington and 

 Vobster " (read in Section C. of the late Meeting of the British 

 Association at Bristol), wherein he showed these isolated patches to 

 have been passed beneath and found separated from any underlying 

 portion of the same limestone, it occurred to me at the time that the 

 structural peculiarities of certain places I had examined would tend 

 to explain those described in the paper, the whole of which I had 

 not the good fortune to hear read, and therefore refrained from 

 offering the following remarks in the Section. 



